The present embodiments relate to a magnetic resonance device having a measuring chamber, a high-frequency shield at least partially enclosing the measuring chamber and an antenna arrangement installed in the magnetic resonance device.
In a magnetic resonance device, a body to be examined may be exposed with the aid of a basic field magnet system to a relatively high basic magnet field of 3 or 7 Tesla, for example. A magnetic field gradient is also applied with the aid of a gradient system. High-frequency excitation signals (HF signals) are emitted using suitable antenna facilities (e.g., suitable antenna devices) by way of a high-frequency transmit system. The nuclear spin in a specific region is tilted about a flip angle defined as an average compared with the magnetic field lines of the basic magnetic field due to this high-frequency field of resonantly excited atoms. This high-frequency excitation and the resulting flip angle distribution are also referred to as nuclear magnetization. As the nuclear spin relaxes, high-frequency signals (e.g., magnetic resonance signals) are emitted. The high-frequency signals are received by suitable receive antennas and are further processed. Raw data thus acquired may be used to reconstruct the desired image data. The high-frequency signals for nuclear spin magnetization may be transmitted by a body coil. The body coil may have the structure of a birdcage antenna that consists of a plurality of transmit rods disposed around the measuring chamber (e.g., the “patient chamber” or “patient tunnel”), parallel to the longitudinal axis of the tomography system. A patient is positioned in the measuring chamber during the examination. On end faces of the antenna rods, the antenna rods are connected capacitively to one another in a ring. The individual transmit rods may consist of conductor tracks equipped with defined reactances (e.g., capacitive elements). The ring segments connecting the transmit rods are in the form of conductor tracks with such reactances. Apart from the ring-type connections on the end faces, it is also possible (e.g., in the case of longer birdcage antennas) for the antenna rods also to be connected in the same manner in a ring shape at one or more points in a central region. The antenna elements of the body coil are disposed directly on a cylindrical tube made of plastic or the like. The cylindrical tube delimits the patient chamber. The antenna elements are attached, in the form of conductor tracks, directly to the tube or to conductor track films covering the tube. The tube may also be referred to as a support tube. The entire arrangement (e.g., the measuring chamber with the antenna arrangement) is enclosed by a high-frequency shield or high-frequency screen that screens the sensitive receive antennas from external interference signals.
The body coil may be used not only to transmit high-frequency pulses but also to receive magnetic resonance signals. Local coils may be used to receive magnetic resonance signals. The local coils are positioned directly on the body of the patient. The local coils may consist of a group or array of conductor loops. The antenna conductor loops may be operated individually. The antenna conductor loops are structured so that the antenna conductor loops may also detect the magnetic high-frequency field of a weak magnetic resonance signal with particular sensitivity. The signals induced in the antenna conductor loop may be amplified, and the amplified signal may be used as raw data after digitization.
Since the overall nuclear magnetization may rotate in the x/y plane (e.g., perpendicular to the longitudinal direction of the measuring chamber referred to as the z direction) in an excited region of the examination object, a magnetization vector for each rotation angle is also essentially perpendicular to the surface of the examination object or patient body. With such antenna conductor loops disposed in a tangential or parallel manner directly on the body surface, the maximum magnetic flux of the magnetic resonance signal is therefore captured. Thus, the maximum possible receive signal may also be induced in the antenna. Such an antenna array may form a relatively large surface antenna on the body of the examination object or patient. A further advantage of such an antenna array with a number of individually operable conductor loops is, for example, that in the context of parallel imaging methods, image acquisition is significantly accelerated, thereby reducing the temporal burden for the patient. Many patients find the positioning of larger local coil arrays on their bodies unpleasant (e.g., patients with claustrophobic tendencies, who feel uncomfortable inside the measuring chamber anyway).
The architecture of a remote body array (RBA) is one way of achieving a magnetic resonance device with a minimum number of local coils.
An antenna arrangement such as an RBA, like an array of local coils, consists of a plurality of individual antenna elements that are not positioned on the examination object or body of the patient but at a distance therefrom. The antenna elements may be located close to walls of the measuring chamber and/or on the outside of the wall of the measuring chamber, as close as possible to the high-frequency screen, in order to keep as much free space available as possible in the measuring chamber. One example of an RBA is described in WO 2007/104607 A1. To save space, the individual conductor loops of the RBA are disposed between the conductor rods of a birdcage antenna used for transmitting. With this arrangement, as with a local coil array, the antenna conductor loops used for receiving are disposed so that a conductor loop surface runs parallel to the high-frequency shield and is therefore also essentially parallel to the body surface of the examination object.
One secondary effect of such an arrangement is that eddy currents induced in the high-frequency shield may cause the magnetic field located orthogonally to the surface of the high-frequency shield to be canceled in proximity to the high-frequency shield and unwanted interference signals to be induced in the receive coils. This reduces the signal-to-noise ratio within an RBA considerably. In order to be able to achieve an adequate air gap between the RBA and the high-frequency shield, either the diameter of the RBA is reduced, with the disadvantage of reducing the diameter of the measuring chamber, or the gradient coil arrangement is enlarged in order to be able to make the high-frequency shield bigger (e.g., also with respect to diameter). If the gradient coil arrangement is enlarged, the linearity and efficiency of the gradient coil arrangement are reduced and the gradient coil arrangement consumes more power.